Battery Runtime Calculator
Enter the battery capacity and voltage and the load in watts or amps. You get the run time, allowing for how much of the battery you can use.
Lead acid batteries last longer if you use only about half of the capacity. Lithium batteries can usually give 80% or more. Cold weather and high loads reduce what you get.
Worked examples
Ideal case
100 Ah, 12 V, 100 W, no losses
12 hours
1,200 Wh / 100 W.
With allowances
100 Ah, 12 V, 100 W, 50% usable, 90% efficient
5.4 hours
1,200 x 0.5 x 0.9 / 100.
About Battery Runtime Calculator
How long will a battery run a light, a fridge or a router? The energy in a battery in watt hours is its capacity in amp hours multiplied by its voltage, and dividing by the power of the load gives the hours. In practice you do not get all of it. A lead acid battery is damaged by being fully discharged, and an inverter that turns battery power into mains wastes part of the energy as heat.
This calculator includes both allowances. Set the usable share to match your battery type, about half for lead acid and 80% or more for lithium, and the efficiency to match any inverter, or set both to 100 for the ideal figure. It also shows the current drawn from the battery, which matters for choosing cable and fuses, and the capacity needed to run the load for one hour. Enter the load in watts, or in amps if that is what you know.
How to use Battery Runtime Calculator
- Enter the battery capacity in amp hours (or milliamp hours) and its voltage.
- Enter the load in watts, or in amps if you know that instead.
- Set the usable share of the capacity and the efficiency of any inverter.
- Read the run time in hours, the usable energy and the current drawn.
Common uses
- Planning a camper van or boat power system
- Sizing a battery for a security camera or router backup
- Estimating how long a portable power station will run a device
- Working out how long a car battery would run a 12 volt fridge
Frequently asked questions
How do I work out battery life in hours?
Multiply the capacity in amp hours by the voltage to get watt hours, multiply by the usable share and the efficiency, and divide by the load in watts. For a 100 Ah 12 V battery and a 100 W load with no losses that is 12 hours.
What usable share should I use?
About 50% for lead acid and AGM batteries if you want them to last, and 80% to 100% for lithium iron phosphate. Check the maker's data for the depth of discharge they recommend.
Why is my real runtime shorter?
Cold weather, an old battery and heavy loads all reduce the capacity you get. Lead acid batteries also deliver less at high currents. Keep a margin of 20% or more.
What efficiency should I use for an inverter?
Good inverters are 85% to 95% efficient at a reasonable load, and much less when lightly loaded. Use 100% if the load runs directly from the battery.